The present invention relates to a data communication method for transmitting high-rate data in addition to low-rate data, and to a data transmission processor and a data reception processor for realizing the data communication method. In the following, the low-rate data means low-volumes of data such as voice data, which may be transmitted at a low rate while the high-rate data means high volumes of data, which are preferably transmitted at a high-rate, other than voice data.
Since traffic for not only voice data but also for other data have been rapidly increased in mobile communications, it is an important consideration to establish a large capacity mobile communication network. Moreover, for a mobile multimedia communication in future, it is preferable that communicators can equivalently treat voice data and other data, and therefore multiplex traffic communications capable of equivalently handling the voice data traffic and other data traffic is required. Such a technique is also required for point-to-point communications.
At present, ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) is standardizing the ATM (Asynchronous Transfer Mode) network system for the above-mentioned multiplex traffic communications. As a result, multimedia communications in the ATM networks including both WANs and LANs can be realized. While the ATM network system is standardized, the standardization of the AAL (ATM Adaptation Layer) between the ATM layer and the higher layer is also carried out. Accordingly, the AAL type 2 protocol in accordance with service class B for transmitting coded data, such as coded voice data, at multiple data rate is standardized in order to lower transmission delay and to enhance transmission efficiency.
By the AAL type 2, protocol low-rate data are transmitted through a plurality of channels provided with a single ATM virtual channel. Since mobile communications mainly treat voice traffic which are sent at low and multiple data rates, the AAL types 2 protocol is superior to use in mobile communications. Therefore, it is preferable that the AAL type 2 protocol is quickly introduced into mobile communications.
However, by the AAL type 2 protocol, it is impossible to transmit user data units having a length of 45 octets or more (one octet equals eight bits). Therefore, if it is applied to multimedia communications in which high volumes of data, which should be usually sent at 64 kilo-bits per second or more, in addition to low volumes of data such as voice data, transmission rate of voice data is delayed because of multiplexing with high volumes of data, so that talking quality may be decayed.
Instead, it would be possible to use the AAL type 5 protocol, which has been already standardized for high-rate data transmission. In this case, a statistical multiplex effect can be obtained because cells for voice data and high-rate data are integrally transmitted at the ATM layer and the lower layer. However, the statistical multiplex effect is limited at the ATM layer and the lower layer, and therefore, the transmission delay of voice data is not always prevented.
Therefore, to realize high-rate data transmission with the AAL type 2 protocol, an improvement is proposed, in which the higher and extended sublayer of the AAL type 2 protocol is equivalent to the AAL type 5 protocol (Goran Eneroth at Ericsson, xe2x80x9cSegmentation proposal for AAL-CUxe2x80x9d, ITU-T Study Group 13 Question 6 Madrid rapporteur meeting, November, 1996). FIG. 8 shows usual hierarchical structure, including the AAL type 2, according to B-ISDN (Broadband aspects of ISDN). As shown in FIG. 8, the high-rate data side of AAL type 2 protocol is divided into two sublayers: the upper one is the service specific convergence sublayer (hereafter abbreviated as SSCS), and the lower one is the common part convergence sublayer (hereafter abbreviated as CPS).
By the above-mentioned proposal, the high-rate data side of the layer structure shown in FIG. 8 is modified into the structure shown in FIG. 9A. That is, in the proposed structure, the SSCS is responsible for a function equivalent to the AAL type 5 protocol. Therefore, as shown in FIG. 9A, when a user data unit 1 exceeding a fixed length (45 octets) is transferred to the SSCS of the AAL type 2 protocol from the higher layer, a restorative code field 2 for error detection and a dummy code field 3 of variable length for padding (adjusting the length) are attached to the user data unit 1, whereby a data unit 4 is assembled in accordance with the AAL type 5 protocol. Next, the data unit 4 is divided into fixed-length segments 5-1 to 5-k (k is an integer equal to or more than 2). Successive information fields 6-1 to 6-k are then attached to the segments 5-1 to 5-k, whereby data blocks 7-1 to 7-k are prepared. Each of the successive code fields 6-1 to 6-k denotes that the corresponding segment is the last one or a successive segment is following. The length of the dummy code field 3 is selected from a range between 0 and 44 octets so that the data length before dividing becomes integral multiples of the fixed length, 45 octets.
The data blocks 7-1 to 7-k are transferred to the CPS of the AAL type 2 protocol. In response, the data blocks 7-1 to 7-k are transformed into CPS packets 8-1 to 8-k by a process according to the AAL type 2 protocol. Next, the CPS packets 8-1 to 8-k are multiplexed and then transferred to the ATM layer. The multiplexed CPS packets are contained ATM cells 9 in the ATM layer, and then the ATM cells 9 are transferred to a physical layer.
By the above-described method, a high-rate data unit is first divided, and then multiplexed in the CPS. Therefore, it is expected that a statistical multiple effect can be also benefited at the CPS that is higher than the ATM layer.
However, by the function equivalent to AAL type 5 protocol in the SSCS, unnecessary data elements, which are not used in the CPS, are included in the data blocks 7-1 to 7-k transferred to the CPS from the SSCS. As shown in FIG. 9B, the restorative code field 2 includes an SSCS-UU (user-to-user) indication field of one octet, a CPI (common part indicator) field of one octet, a length indicator field of two octets, and an error correcting code field or cyclic redundancy check(CRC) field of four octets. However, the AAL type 2 protocol does not necessarily utilize some of them. Consequently, the redundancy of transmitted data is high in comparison with the communications simply using the AAL type 5 protocol and the AAL type 2 protocol in parallel, and transmission efficiency may be decreased.
On the other hand, the layer higher than the AAL is utilized typically for managing voice communication and data communication. In voice communication, it is not always necessary that all of data elements be transmitted as long as a certain quality is secured, but the information should be sent in real time. On the other hand, in data communication, all of the data elements must be transmitted accurately although quickness is not needed very much. Therefore, if a data error is found, it is necessary to retransmit in data communication.
Thus, it is the function of the higher layer to decide the necessity of retransmission whether or not the lower layer is the AAL type 2 protocol. When the retransmission is necessary, one or more data units (e.g., user frames) adapted to the higher layer are retransmitted from the higher layer. The user frame length adapted to the higher layer usually ranges between 256 octets and 64 kilo-octets.
On the other hand, the frame length should be small for transmission at the lower layers. For example, the frame length is 45 octets or less in accordance with the AAL type 2 protocol. Therefore, it is necessary to divide a user data frame from the higher layer into a plurality of segments before transferring them to the lower layers. However, since one or more user frames, which are longer, are retransmitted from the higher layer if a data error occurs, transmission efficiency may be decreased.
Accordingly, it is an object of the present invention to provide a data communication method for enhancing transmission efficiency, and to provide a data transmission processor and a data reception processor for realizing the method.
To solve the above problems, in one aspect of the present invention, the method comprises the steps of: dividing a data unit into a plurality of packets; attaching identifiers to the packets; transmitting the packets; receiving the packets; deciding whether all of the packets for reassembling the data unit are received in accordance with the identifiers; and reassembling the user data unit when the decision is affirmative.